Internal combustion engine pistons are exposed to extremely tough working environments. They are subjected to high temperatures, explosive firing pressures, side forces and inertial forces. As an engine's output is increased more and more, temperatures, cylinder pressures and engine speed can become so high that traditional materials from which pistons are made, including aluminum alloys, reach their fatigue strengths.
Articulated pistons are two-piece pistons that have a crown made of steel and a skirt made from aluminum. The crown and skirt are joined together by means of a piston pin. In articulated pistons, the crown and skirt are able to articulate so as to move independently of each other.
Articulated pistons provide several advantages over one-piece cast aluminum pistons. For example, the steel crown in articulated pistons has a thermal expansion rate that is more similar to the thermal expansion rate of iron piston liners than aluminum. Heat from the steel crowns of articulated pistons is not as easily transferred to the aluminum skirt so the skirt retains its shape better.
Although articulated pistons can withstand relatively higher pressures and temperatures there are some practical design limitations associated with articulated pistons. For example, since articulated pistons require longer piston pins, making the total piston assembly (piston plus piston pin) generally heavier than one-piece aluminum piston assemblies. In addition, since the piston crown and skirt move independently of each other the skirt cannot effectively function to guide movement of the piston crown. Accordingly, the piston land has to guide movement of the piston crown. This results in land to cylinder liner contact which can cause cavitation problems. Another design limitation associated with articulated pistons is that there is no connection between the ring belt and skirt. This allows stresses to be very high in the cooling gallery and on the bowl edge, which can cause cracks to occur. Moreover, the lack of connection between the ring belt and skirt and resulting stresses allow for ring groove deformations to be very high which can cause oil consumption, blow-by, and emission problems.
Piston designers have been trying very hard to come up with new technologies to overcome the problems associated with articulated pistons. A number of proposed solutions have focused on one-piece steel pistons. Unlike articulated piston, the skirt and crown of one-piece steel pistons form an integrated unit with the piston crown having a cooling gallery in it. Examples of patented one-piece steel pistons are found in DE 44 46 726 A1 to Kemnitz, U.S. Pat. No. 6,223,701 to Kruse, EP 0 992 670 A1 to Gaiser et al., and International Application Publication No. WO 01/50042 to Gaiser et al.
One of the most challenging aspects of one-piece piston designs is creating a cooling gallery in the piston crown while at the same time ensuring sufficient margins for fatigue strength and minimizing ring groove deformations subject to loads. In DE 44 46 726 A1 the piston is not connected between ring belt and skirt. Therefore, the overall structure of the piston is not stable and high stress can cause deformation to occur in the piston crown. In addition, because the skirt of the piston is short in DE 44 46 726 A1, high contact pressures will be created between the skirt and cylinder liner. Moreover, the shortness of the skirt used in DE 44 46 726 A1 limits the ability of the skirt to guide the movement of the piston so that cavitations can occur with respect to the cylinder liner. Overall, the process of manufacturing the one-piece piston of DE 44 46 726 A1 is very intensive.
WO01/50042 A1 involves joining upper and lower crown sections by a friction weld. The friction welding used in this piston design changes the original material properties. Moreover, cracks can occur in the welding areas either during welding or subsequent heat treatment or heating. In addition, because welding flashes in a cooling gallery cannot be removed they will reduce the effective cooling gallery volume and could, in a worst case scenario, block the cooling gallery completely. Further, as a result of friction welding, metal particles remaining in the cooling gallery could damage an engine if they are released from the cooling gallery while the engine is running.
The present invention is directed to a one-piece steel piston that is made from a piston blank that is provided with a portion that configured and designed to be displaced to form a cooling gallery and ring belt.